It is highly active at all of the muscarinic receptors, but has little effect on thenicotinic receptors. Methacholine has a chargedquaternary amine structure, rendering it insoluble in lipidcell membranes. Clinically, this means that it will not cross theblood–brain barrier and has poor absorption from thegastrointestinal tract. It is broken down at a relatively slow rate within the body, due to its relative resistance toacetylcholinesterases.
The chemical structure of methacholine is derived fromacetylcholine by the substitution of a methyl group on theβ carbon; this methyl group increases selectivity formuscarinic receptors overnicotinic receptors. The quaternary ammonium group is essential for activity.
Use of methacholine is contraindicated in patients with recent heart attack or stroke, uncontrolled hypertension, known severe airway disease, or aortic aneurysm. It may be used with caution by nursing or pregnant mothers and patients taking certain medications formyasthenia gravis.[2]
^Birnbaum S, Barreiro TJ (June 2007). "Methacholine challenge testing: identifying its diagnostic role, testing, coding, and reimbursement".Chest.131 (6):1932–5.doi:10.1378/chest.06-1385.PMID17565027.
^Valentin Popa (2001). "ATS guidelines for methacholine and exercise challenge testing".American Journal of Respiratory and Critical Care Medicine.163 (1):292–293.doi:10.1164/ajrccm.163.1.16310b.PMID11208661.
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